This invention relates to AM stereophonic reception and, more particularly, to the provision of a gradual blend function in response to certain signal conditions.
In the AM stereophonic system with which this invention was designed to function, the transmitted and received signal can be represented by the following formula: EQU (1+L+R) cos (w.sub.c t+.phi.)
where L and R are information signals, w represents the carrier frequency and .phi. is the angle whose tangent is [(L-R)/(1+L+R)]. This signal and the system employing it are described in U.S. Pat. No. 4,218,586, assigned to the same assignee, and incorporated herein by reference. For purposes of the present invention, it is only necessary that the signal as received and detected requires the use of a correction signal in order to restore the original L and R signals. L and R, it will be understood, represent any two information signals which it is desired to transmit on one carrier signal. In common usage they represent left and right stereophonic signals such as would be produced by two separated microphones or the like.
In the prior art, particularly in the field of FM stereo reception, there are many different circuits for providing a change from stereophonic to monophonic reception when the received signal is less than satisfactory. In FM broadcasting, the need for a blend function arises, for example, when the transmitted signal is severely affected by multipath reception or by interference from other stations, becoming exceptionally noisy and/or distorted. Since FM stereo signals inherently suffer a 23 db degradation as compared to monophonic signals, a weak signal having a significant amount of noise would make monophonic reception more desirable than stereo. In U.S. Pat. No. 3,825,697, a blend control signal is derived by examining the phase error between the 38 kHz demodulating signal and the 19 kHz pilot signal, the control signal establishing a connection between the L and R output terminals when the phase error exceeds a predetermined value. The control signal provides for a rapid change to monophonic operation and a gradual change back to stereophonic operation.
In another patent, U.S. Pat. No. 4,379,208, a blend control is combined with an AM stereo pilot signal indicator in a receiver wherein blending is a function of loss of pilot signal, receiver mistuning, or an excessively weak received signal.